Free-space optical communication module for small satellites

ABSTRACT

Communication bottlenecks, particularly in the downlink direction, are a common problem for many CubeSat developers. As described herein, a CubeSat module for a CubeSat comprises an optical transmitter to transmit data to a remote terminal, a receiver to acquire an optical beacon from a remote terminal, and a fine-pointing module operably and directly coupleable to a coarse-pointing module of the CubeSat. The fine-pointing module is configured to point the optical transmitter toward the remote terminal with an accuracy range that overlaps with an accuracy range of the coarse-pointing module of the CubeSat so as to establish a communications link between the CubeSat and the remote terminal over a low-Earth-orbit (LEO) distance.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation application of U.S. application Ser.No. 14/817,985, filed Aug. 4, 2015, and entitled “Design of a Free-SpaceOptical Communication Module for Small Satellites,” which claimspriority, under 35 U.S.C. §119(e), from U.S. Application No. 62/033,321filed Aug. 5, 2014, and entitled “Design of a Free-Space OpticalCommunication Module for Small Satellites,” and also claims priority,under 35 U.S.C. §119(e), from U.S. Application No. 62/112,854 filed Feb.6, 2015, and entitled “Design of a Free-Space Optical CommunicationModule for Small Satellites,” which applications are hereby incorporatedherein by reference in their entireties.

GOVERNMENT SUPPORT

This invention was made with Government support under Contract Nos.NNN12AAO1C and NNX13AM69H awarded by the National Aeronautics and SpaceAdministration. The Government has certain rights in the invention.

BACKGROUND

Miniaturized satellites such as CubeSats continue to improve theircapabilities to enable missions that can produce significant amounts ofdata. For most CubeSat missions, data must be downlinked during shortlow-earth orbit ground station passes, a task currently performed usingtraditional radio frequency (“RF”) systems.

SUMMARY

Embodiments of the present invention include a CubeSat module for aCubeSat. In some embodiments, the CubeSat module (also referred toherein as a “payload”) includes an optical transmitter to transmit datato a remote terminal, a receiver to acquire an optical beacon from aremote terminal, and a fine-pointing module operably and directlycoupleable to a coarse-pointing module of the CubeSat. The fine-pointingmodule is configured to point the optical transmitter toward the remoteterminal with an accuracy range that overlaps with an accuracy range ofthe coarse-pointing module of the CubeSat so as to establish acommunications link between the CubeSat and the remote terminal over alow-Earth-orbit (LEO) distance.

In some embodiments, a CubeSat comprises a beacon receiver, acoarse-pointing module, an optical transmitter, and a fine-pointingmodule operably coupled to the coarse-pointing module. According to onesuch embodiment, a method of free-space optical communication for theCubeSat includes pointing the beacon receiver with the coarse-pointingmodule toward a terrestrial terminal with an attitude accuracy of withinabout 3°, and acquiring a beacon from the terrestrial terminal. Inresponse to acquiring the beacon, the coarse-pointing module points thebeacon receiver toward the terrestrial terminal with an attitudeaccuracy of within about 1°. The fine-pointing module points a beamemitted by the optical transmitter toward the terrestrial terminal witha pointing accuracy of about 0.03°, thereby establishing an opticaldownlink between the CubeSat and the terrestrial terminal.

In some embodiments, a CubeSat includes a beacon receiver to acquire anoptical beacon emanating from a terrestrial terminal. The CubeSat alsoincludes a coarse-pointing module to align the beacon receiver with theoptical beacon over a first accuracy range prior to acquisition of theoptical beacon by the beacon receiver and to align the beacon receiverwith the optical beacon over a second accuracy range in response toacquisition of the optical beacon, the second accuracy range beingsmaller than the first accuracy range. The CubeSat also includes anoptical transmitter to transmit data to the terrestrial terminal, and afine-pointing module that is operably coupled to the coarse-pointingmodule, and configured to point a beam emitted by the opticaltransmitter toward the terrestrial terminal with an accuracy range thatoverlaps with the second accuracy range of the coarse-pointing module.

In some embodiments, a CubeSat includes a coarse-pointing module topoint the CubeSat toward a remote terminal with a first accuracy range.According to one such embodiment, a CubeSat module for a CubeSatincludes an optical transmitter to transmit data to the remote terminal,a receiver to acquire an optical beacon, and a fine-pointing module. Thefine-pointing module is operably coupled to the coarse-pointing module,and is configured to point an output of the optical transmitter towardthe remote terminal with a second accuracy range at least partiallyoverlapping the first accuracy range. The fine-pointing module includesa microelectromechanical systems (MEMS) micromirror, disposed in anoptical path of the output of the optical transmitter, to align theoutput of the optical transmitter with respect to the optical beacon.The fine-pointing module also includes an interface to transmit finepointing information to and to receive coarse pointing information fromthe coarse-pointing module of the CubeSat.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1 is a rendering of a satellite communication system according tosome embodiments.

FIG. 2 is a block diagram of a satellite communication system accordingto some embodiments.

FIGS. 3A-3C are renderings of a physical layout, including perspective,top and side views, respectively, of a communications payload, occupyinga 0.5 U volume of a CubeSat, according to some embodiments.

FIGS. 4A-4C show a pointing, acquisition and tracking (“PAT”) process,according to some embodiments.

FIG. 5 is a rendering of a satellite communication system according tosome embodiments.

FIG. 6 is a rendering of a first step in a PAT process implemented usingthe system of FIG. 5.

FIG. 7 is a rendering of a second step in a PAT process implementedusing the system of FIG. 5.

FIG. 8 is a rendering of a third step in a PAT process implemented usingthe system of FIG. 5.

FIG. 9A is a rendering of a closed-loop tracking configuration using aquadcell tracking detector, according to some embodiments.

FIG. 9B is a rendering of a closed-loop tracking configuration using afocal plane tracking detector, according to some embodiments.

FIG. 10 is a system block diagram showing components of a ground segmentaccording to some embodiments.

FIG. 11 is a block diagram showing components of a high power laserdiode, according to some embodiments.

FIG. 12 is a block diagram showing components of a master oscillatorpower amplifier (MOPA), according to some embodiments.

FIG. 13 is a diagram showing a linear single-axis model of an attitudecontrol system, according to some embodiments.

FIG. 14 is a flow-down diagram of key parameters of attitudedetermination and control systems, according to some embodiments.

FIG. 15 is a flow-down diagram of constraints and parameters of attitudedetermination and control systems, according to some embodiments.

FIG. 16 is a transmitter test configuration according to someembodiments.

FIG. 17 is a plot of wavelength versus DC laser bias, with each curvecorresponding to a different seed laser temperature, according to someembodiments.

FIG. 18 is a plot of power consumption versus temperature for anexemplary transmitter, according to some embodiments.

FIG. 19 is a plot of insertion loss versus frequency, comparing 5 GHzand 10 GHz passband filters, according to some embodiments.

FIG. 20 is a plot showing extinction ratio measurements for a seed laserwith an extinction filter, according to some embodiments.

FIGS. 21A-21F show a sequence of system components and relatedelectrical and optical input/output waveform plots, according to someembodiments.

FIGS. 22A and 22B are bode plots showing the frequency response of afine steering mirror, with and without a Bessel filter, respectively,according to some embodiments.

FIGS. 23A and 23B show power/performance plots for erbium-doped fiberamplifiers (“EDFAs”), according to some embodiments.

FIG. 24 is a plot showing atmospheric refractive index structureparameter profiles for a stationary beam and for a beam with 1°/s slewspeed, according to some embodiments.

FIGS. 25A and 25B are images simulating the identification of a beaconregion of interest, according to some embodiments.

FIGS. 26A and 26B are plots showing beacon acquisition simulationresults, according to some embodiments: FIG. 26A is a plot of fadeprobability for various transmit laser power levels; and FIG. 26B showsthe percentage pointing accuracy during centroiding.

FIG. 27 is a plot showing attitude error for systems using a coarsepointing stage with and without a fine pointing stage, according to someembodiments.

DETAILED DESCRIPTION

Communication bottlenecks, particularly in the downlink direction, are acommon problem for many CubeSat developers. Radio frequency solutionshave poor link power efficiency (joules per bit), may be limited byantenna gain, and often carry complex regulatory burdens. Most CubeSatsare in LEO and have fairly short ground station access times (<10min/pass). CubeSats often use low-rate ultra-high frequency (UHF) links,with data rates for amateur bands of ˜1200 bps and for industrial,scientific and medical (“ISM”) radio bands of <115 kbps. High-rate radiofrequency (RF) commercial off-the-shelf (COTS) products are availablefor UHF and S-band communications, but very large ground apertures aretypically required. Furthermore, the pointing accuracy required for ahigh bandwidth downlink exceeds the capabilities of traditionalCubeSats. The current state-of-the-art in demonstrated CubeSat absolutepointing accuracy ranges from 1-5° RMS. However, to achieve a 10-50 Mbpslink within the power constraints of a typical CubeSat, a finer pointingaccuracy is required.

Systems of the present disclosure comprise an optical transmitter totransmit data to a remote terminal, a receiver to acquire an opticalbeacon from a remote terminal, and a fine-pointing module operably anddirectly coupleable to a coarse-pointing module of the CubeSat. Atwo-stage control approach to pointing, acquisition and tracking (“PAT”)is used, in which coarse body pointing of the CubeSat (e.g., the hostattitude determination and control system, “ADCS”) is augmented with afast-steering mirror (FSM) for fine control (a “coarse” stage and a“fine” stage, respectively). In some embodiments, a free-space optical(“FSO”) communications system is “asymmetric,” in that it includes botha high-rate optical downlink (“DL”) and a low-rate RF link (“UL”), aswell as an optical beacon for acquisition and tracking. CubeSat FSOcommunication payloads described herein are designed to be compatiblewith a typical 3-axis stabilized CubeSat, and the system architecture ofthe disclosure takes into account the fact that many operators of FSOcommunications systems need a high-rate downlink (e.g., for remotesensing).

FSO communications systems of the disclosure provide functionality forday and/or night operation, with better access and throughput than haspreviously been possible. In some embodiments, sun sensors are used forattitude determination. In some embodiments, the PAT system does not usea 2 axis electromechanical gimbal. PAT systems of the disclosure canimprove coarse pointing by about 4 orders of magnitude as compared withprior methods. Fast beam steering can be performed usingmicroelectro-mechanical systems (“MEMS,” e.g., fast-steering mirrors,micro-mirror arrays, etc.), acousto-optical methods, optical phasedarrays, and/or the like.

In some embodiments, an incoherent (direct) satellite FSO communicationssystem includes a coarse stage pointing subsystem, a fine stage pointingsubsystem, and a beacon acquisition subsystem. The coarse stage pointingsubsystem has an accuracy range of +/− about 5 degrees when “unlocked”with respect to a beacon, and +/− about 1.25 degrees or +/− about 1degree when locked to the beacon. The fine stage pointing subsystem hasan accuracy range of ˜+/−1 degree. The accuracy range of the coarsestage can overlap with the accuracy range of the fine stage. In someembodiments, the coarse stage subsystem and the fine stage subsystemhave accuracy ranges that overlap the improved uncertainty range (e.g.,after acquisition of a beacon and the corresponding improvement insatellite position knowledge). Collectively, the two-stage pointingcontrol mechanism can achieve a pointing performance of ±0.09 mrad 3-σwithout bias, sufficient for a 2.1 mrad downlink laser.

In some embodiments, terminal designs of the disclosure are budgeted fora volume of 10 cm×10 cm×5 cm (i.e., smaller than a standard CubeSat,which is 10cm×10cm×10cm, also referred to as “1U”), a weight of <1 kg,and a power of <10 W, while delivering a user data rate of 10 Mbps to 50Mbps—a full order of magnitude improvement over prior RF solutions. Insome embodiments, commercial off-the-shelf (COTS) components are used.CubeSats typically have short missions (<1 year) in low Earth orbit(LEO) where radiation and thermal stresses are relatively benign.

CubeSat FSO Communications System

FIG. 1 is a rendering of a satellite communication system according tosome embodiments. As shown in FIG. 1, a CubeSat 100 acquires an opticalbeacon B for acquisition and tracking, a low-rate radio frequency (RF)link (uplink and downlink) is established between an RF station 102 andCubeSat 100, and a high-rate optical downlink is established from theCubeSat 100 to an optical station 104. The RF station 102 and theoptical station 104 are positioned at one or more ground stations. Insome embodiments, potential channel impairments such as cloud cover canbe mitigated through strategic placement of ground stations (e.g., inareas of favorable weather) and/or through the use of onboard storagememory. The CubeSat contains a payload comprising a two-stage pointingsystem, having a coarse pointing module (providing functionality for acoarse pointing “stage,” e.g. comprising a host ADCS), and a finepointing module (providing functionality for a fine pointing “stage,”e.g. using an integrated fine-steering mirror (“FSM”)).

During a communications pass, the CubeSat 100 uses an on-boardpropagated orbit to point towards the ground station and wait for anuplink beacon. The field-of-view of a beacon camera on the CubeSat 100is selected to cover the entirety of the uncertainty region so that noadditional maneuvers are needed to search for the ground station. Oncethe CubeSat beacon camera detects the beacon signal from the groundstation, it uses this information to improve the pointing accuracy towithin the range of the fine stage. Finally, the FSM steers the transmitbeam to the accuracy desired for downlink. The FSM is in a bistaticconfiguration, so there is no feedback on the position of the FSM. Anon-orbit calibration procedure can be performed to ensuretransmitter/receiver alignment. Such calibration can utilize thelow-rate RF link to communicate the received power measurements on theground back up to the satellite. Using this feedback, the satellite canadjust its pointing until peak power is received on the ground.

FIG. 2 is a block diagram of a satellite communication system accordingto some embodiments, showing interactions between a host spacecraft 200and a lasercom payload 203. The lasercom payload 203 includes high-speedelectronics 207 (to perform, for example, data encoding, interleaving,modulation, and/or framing) that swap data and control commands with ahost on-board computer 205 of the host spacecraft 200 (e.g., a CubeSat),a low-rate radio modem 209 (configured for communication via antenna“A”) and a telemetry buffer 211 each electrically coupled to thehigh-speed electronics 207, as well as to an optical assembly 213. Theoptical assembly 213 of the lasercom payload 203 includes a transmitlaser and modulator (collectively 215), the transmit laser beingoptically coupled to at least one microelectromechanical system (“MEMS”)fine-steering mirror (“FSM”) 217 for fine pointing of the hostspacecraft 200. The MEMS FSM 217 is, in turn, optically coupled to atransmit (“TX”) aperture 219, through which the transmit beam passesduring operation. The transmit laser and modulator 215 are electricallycoupled to, and driven by, the high-speed electronics 207 (e.g., forencoding, interleaving, modulation, framing, etc.). A receiver (“RX”)aperture 221 (e.g., having a diameter of about 2.5 cm), configured toreceive an incoming optical transmission (e.g., a beacon or beam), isoptically coupled to a tracking detector 223 (e.g., comprising a focalplane array) whose output is fed into a positioning, acquisition andtracking (PAT) module 225 that applies one or more PAT algorithms to thedetector data (e.g., for centroiding and/or feedback control by aproportional-integral-derivative (“PID”) controller). The PAT module 225can provide a boresight offset estimate to high voltage (“HV”) driveelectronics 227 that drive the MEMS FSM. The PAT module 225 can alsoelectrically transmit coarse corrections (e.g., for body pointing) to anADCS subsystem 229 of the host spacecraft for coarse pointing and/orslew. In some embodiments. The optical assembly 213 includes at leastone actuator, operably coupled to the at least one MEMS FSM (or“micromirror”), to actuate at least one micromirror about two axes.

FIGS. 3A-3C are renderings of a physical layout, including perspective,top and side views, respectively, of a spacecraft laser communicationspayload 306, occupying a 5 cm×10 cm×10 cm (“0.5U”) volume of a 3UCubeSat 300 and having coarse and fine stage pointing capability,according to some embodiments. The lasercom payload 306 includeshigh-speed electronics 307 to control focal-plane readout (e.g., oftracking focal plane 335), centroiding of a received beacon, forwarderror correction (“FEC”), interleaving (e.g., to mitigate atmosphericfading so as to reduce an error rate associated with transmitting datavia the communications link), framing, and modulation. The lasercompayload 306 comprises two main subsystems: a downlink transmitter and anuplink beacon receiver. The lasercom payload 306 uses a bi-staticdesign, with separate downlink and uplink beacon paths through downlinkaperture 341 and uplink aperture 343, respectively. A sun blockingfilter 339 and a narrow filter 337 are disposed in the uplink path.

The transmitter design follows a Master Oscillator Power Amplifier(MOPA) architecture, where an Erbium Doped Fiber Amplifier (“EDFA”) 345is used in conjunction with a 1550 nm seed laser 310 to provide a highpeak-to-average power optical waveform. EDFAs are widely available dueto their use in the telecommunications industry. The industry standard“MSA” form-factor for an EDFA is (9 cm×6 cm×1.5 cm), which can fitwithin a 10 cm×10 cm chassis cross-section. EDFAs are offered in avariety of power output levels and gains (e.g., 200 mW optical). In someembodiments, mechanical modifications are made to the EDFA so that fiberegress points are located along a “long edge” of the CubeSat chassis toease fiber routing within the CubeSat chassis. The EDFA can comprise amodified COTS fiber amplifier (e.g., from NuPhoton Technologies, Inc.)with a form factor of about 9 cm×7 cm×1.5 cm, and can be configured forelectrical input of 5V at 5.7W and have an optical output of 200 mWavg., a gain of 40 dB and a “wall plug” efficiency of ˜3.5%.

A fiber collimator 333 forms the transmit beam, which is subsequentlydirected by a fine-stage fast steering mirror (FSM) 317 (e.g., driven bythe fine-steering mirror driver 331) in a “gimballed-flat” topology. TheFSM 317 can be a SWaP-compliant MEMS tip/tilt mirror having a steeringrange (e.g., of +/− about1°) that is sufficient for coarse stagehand-off. This hand-off can be autonomous and can be based on qualitymetrics comprising one or more of the following: beacon signal-to-noiseratio (“SNR”), beacon transmitter power, receiver power, and attitudestability of the coarse stage. The transition could also be driven by aground command, either manually or in response to detection of thedownlink signal. Once the readouts from the beacon receiver meet someconfidence criteria (e.g., a reliable bright signal across multipleframes, or a signature that matches a known modulation scheme), the finestage begins steering. The beacon receiver camera comprises a CMOS focalplane array with high sensitivity in the near-infrared (NIR) range todetect an 850 nm beacon transmitted from the ground station. The uplinkbeam image can be processed using centroiding algorithms for fineattitude determination.

Example System Design Parameters

Tables 1-12 (below) provide design parameters of an FSO communicationssystem, according to some embodiments. The ground segment can include atransportable telescope and mount (e.g., 30 cm) and can use COTSdetector technology (e.g., avalanche photodiode detectors (APD), PINphotodiode, etc.). In some embodiments, a downlink radiometry involves a1550 nm (at 1 W (optical)) transmitter, a ground segment downlinkreceiver with a ˜30 cm aperture and a sensitivity of about 1000photons/bit, and channel/pointing losses of ˜6dB. In some embodiments,the half-power beamwidth needs to be 0.12 deg to achieve 10 Mbps and theFSO pointing capability is about 1/10^(th) the beam width (0.012 deg or0.21 mrad or 0.72 arcmin).

TABLE 1 Example Top-Level Design Parameters Link Parameters Link rate 10Mbps, 50 Mbps Uncoded channel rate Bit error rate 10⁻⁴, (e.g., usingcode) Conservative baseline for FEC Range 1000 km (400 km LEO orbit) 20°elevation above horizon @ 400 km LEO Space Segment Parameters Size,Weight 10 cm × 10 cm × 5 cm, 1 kg “0.5 U” CubeSat mid-stack payloadPower 10 W (transmit), 1 W (idle) Excludes host ADCS Coarse Pointing 5°(3-sigma), 1°/sec slew Host CubeSat ADCS Fine Pointing 0.1 mrad(3-sigma) FSO Payload fast- steering mirror Downlink Beam 1550 nm, 2.1mrad (0.12°) FWHM divergence divergence Beacon Receiver Uncooledfocal-plane array 850 nm (TBR) Ground Segment Parameters Apertures RX:30 cm, beacon: TBD Mount capable of tracking LEO object Acq. DetectorInGaAs Camera Informs tip/tilt FSM Comm. Detector COTS APD/TIA ModuleCooled module Pointing Coarse: Reaction wheel(s), Detector size demandsmagnetorquer(s), and/or fine stage two-line element(s) (“TLE”), Fine:tip/tilt FSM (e.g., 2-axis)

TABLE 2 Example Top-Level Design Parameters Link Parameters Data rate 10Mbps, 50 Mbps (stretch) User data rate Bit error rate 10{circumflex over( )}−4 without coding Conservative baseline for FEC (7% RS planned) Pathlength 1000 km ~20 deg elev @ 400 km LEO Space Segment Parameters Size,Weight 10 × 10 × 5 cm, 1000 g “0.5 U” CubeSat mid-stack payload Power 10W (transmit), 1 W (idle) Excludes host ADCS Coarse Pointing +/−3°(3-sigma), 1°/sec slew Host CubeSat ADCS Fine Pointing +/−0.03° (+/−525urad) 3-sigma Lasercom payload fast-steering mirror Downlink Beam 1550nm 0.12° (2.1 mrad) FWHM Radiometric constraint for 10 Mbps BeaconReceiver Uncolled Si focal-plane array 850 nm Ground Segment ParametersApertures RX: 30 cm, beacon: TBD Mount capable of tracking LEO objectAcq. Detector InGaAs Camera Informs tip/tilt FSM Comm. Detector COTSAPD/TIA Module Cooled module, link operating at 300 photons/bit PointingCoarse: TLE, Fine: tip/tilt FSM Detector size demands fine stage

TABLE 3 Example Design Parameters Value Optics Focal Length 25 mmCentroid Error 0.018 mrad RMS Reaction Wheel (MAI-400) Max. Torque 635mNm Resolution 0.005 mNm Gyroscope (ADIS-16534) Angular Random Walk 2deg/√hr Output Noise 0.75 deg/s RMS Resolution 0.0125 deg/s SpacecraftTrue Moment of Inertia 0.05 kgm{circumflex over ( )}2 Est. Moment ofInertia 0.0475 kgm{circumflex over ( )}2

TABLE 4 Example Top-Level Design Parameters Notes Optical LinkParameters Optical link rate 10 Mbps (goal) Uncoded channel rate 50 Mbps(stretch) Bit error rate 1 × 10⁻⁶ BER Without coding Operational range≤1000 km Appropriate for most LEO missions Optical Space SegmentParameters Size 0.5 U 5 cm × 10 cm × 10 cm Mass 2 kg Power 10 W (TX)Includes FSO payload, 1 W (idle) excludes host ADCS PAT schemeclosed-loop Using uplink beacon Coarse pointing satellite body-pointingProvided by host ADCS goal: 2.0° (TBR) 3σ, absolute stretch: 0.5° (TBR)3σ, absolute Coarse slew rate 3.0 deg/s (TBR) Provided by host ADCS Finepoint/track single two-axis MEMS Shared by TX and RX op- tical pathsFine point range ±5.0° (TBR) Greater than coarse pointing accuracy Finepoint resolution TBD Beam width dependent Ground Segment ParametersReceive aperture ≤30 cm COTS telescope Mass 50 kg For portability PATscheme open-loop Based on TLE/epbemeris Detector APD, PMT, etc.Commodity/COTS unit is desirable Uplink beacon TBD eye-safe laser Req.for closed-loop tracking

TABLE 5 Link and Module Parameters Link parameters Data rate 10-50 MbpsBit error rate 10⁻⁴ (no coding) Conservation baseline Path length 1000km (at 20° elevation) LEO orbit at 400 km altitude NODE module Size,weight 10 × 10 × 5 cm, 0.6 kg 0.5 U CubeSat Power 10 W (transmit)CubeSat constraints Downlink 0.12° FWHM Provide required data beam rateBeacon Silicon array, 7° FOV COTS components, cover receiver coarsepointing range Coarse pointing +/−3° (3-σ) Host CubeSat ADCS Finepointing +/−0.03° (3-σ) Fast-steering mirror

TABLE 6 Example FSO Payload Mechanical Parameters ID Parent DescriptionMECH-1 The FSO payload shall fit within a 0.5 U (10 × 10 × 5 cm) volumeenvelope MECH-2 The FSO payload shall have a mass no greater than 1 kgMECH-3 The FSO payload shall have “side-looking” so that it can besituated in the midsection of the CubeSat MECH-4 The FSO payload'sbistatic apertures shall maintain alignment within X mrad (TBD) acrossexpected environmental disturbances (thermal gradients, vibe, shock)

TABLE 7 Example FSO Payload Electrical Parameters ID Parent DescriptionELEC-1 The FSO payload shall consume no more that 1 W (idle) and 10 W(during TX) ELEC-2 The FSO payload shall accept unregulated bus voltagesbetween 6 V and 10 V

TABLE 8 Example Communication Link Parameters ID Parent DescriptionCOMM-1 The optical dowlink shall provide 10 Mbps (goal), 50 Mbps(stretch) user information rate COMM-1.1 COMM-1 The optical downlinkshall operate at channel BER less than 10{circumflex over ( )}−4COMM-1.2 COMM-1 The FSO payload shall provide FEC COMM-1.3 COMM-1 TheFSO payload shall provide interleaving to mitigate atmospheric fadingCOMM-2 The optical downlink beam divergence shall be 2 mrad (FWHM)

TABLE 9 Example Ground Station Parameters ID Parent Description GND-1The ground station telescope(s) shall be capable of continuouslytracking a LEO object without entering gimbal lock GND-2 The groundstation shall provide an uplink beacon signal GND-2.1 GND-2 The beaconsignal shall not be visible or listed as a dis1raction hazard. GND-2.2GND-2 The beacon signal shall be eye-safe (i.e. below MPE) at the beacontransmit aperture GND-2.3 GND-2 The beacon beam divergence shall belarge enough to ensure 3-sigma probability of illumination givensatellite position uncertainties GND-3 The ground station shall providea 30 cm (TBR) aperture for receiving the downlink communication signalGND-4 The downlink receiver shall use COTS detector technology such asAPD/TIA modules GND-5 The ground station shall incorporate a wide FOVacquisition sensor GND-5.1 The ground station shall provide a means tocalibrate alignment of acquisition sensor and communication detector

TABLE 10 Example Transmitter, Channel and Receiver Parameters SymbolValue Units Notes Transmitter Parameters Laser optical output power PLD,elec 1 W Laser wavelength >..peak 1550 nm Peak wavelength Modulationduty cycle 0.5 Simple RZ for now Laser avg. optical power (dBW) PLD,opt, avg, d B −3.0 dBW Half-power beam width θ½ 0.120 degrees Full coneangle where power is half of peak intensity Transmit antenna gain (dB)Gt, dB 65.60 dBi Based on divergence above Channel Parameters Pathlength d 1000 km LEO at 400 km can be tracked down to 20 degrees abovehorizon Path loss (dB) Lpath, dB −258.2 dB Standard free-space path lossequation Atmpheric loss placeholder Latm, dB −6.00 dB Placeholder valuefor absorbtion, scattering, turbulence Receiver Parameters Aperturediameter 30 cm Receive antenna gain (dB) Gr, dB 115.7 db Diffractionlimited gain Power at detector (dB) Prec, dB −85.9 dBW Power at detectorPree 1.28E−09 W or J/s Photons per second 1.00E+10 photons/sec Requiredphotons/bit 1000 photons/bit An ″easy# to achieve receiver sensitivityPredicted data rate 10000102 bits/sec

TABLE 11 Example Power Budget Breakdown TX UL_ACQ IDLE Comm Search/DL_ACQ Accept, encode, Mode: downlink acquire UL Track UL beacon, storetelemetry Nominal in progress beacon transmit DL acq. from host PowerDuty Duty Duty Duty Component (W) (%) P_avg (%) P_avg (%) P_avg (%)P_avg Focal plane array FPA quiescent 0.05 100% 0.05 100% 0.05 100% 0.05FPA readout 0.11 100% 0.11 100% 0.11 100% 0.11 power PAT Processor 0.25100% 0.25 100% 0.25 100% 0.25 Fast steering 0.25 100% 0.25 100% 0.25mirror + driver High-Speed 0.25 100% 0.25 100% 0.25 Electronics FECencoder 0.25 100% 0.25 100% 0.25 100% 0.25 Non-vol telem. 0.25 100% 0.250.25 100% 0.25 100% 0.25 buffer (SSR) Modulator/ 3.00 100% 3.00 100%3.00 framer Laser transmitter EDFA 3.00 100% 3.00 100% 3.00 Seed laserdriver 0.25 100% 0.25 100% 0.25 Seed laser TEC 0.25 100% 0.25 100% 0.25100% 0.25 Radio Modem Receive only 0.10 95% 0.10 50% 0.05 50% 0.5 100%0.10 Transmit/receive 1.50 5% 0.08 50% 0.75 50% 0.75 Mode Total (W) 7.581.71 8.21 0.60 Mode Budget (W) 10.00 10.00 10.00 1.00 Power Margin (%)24% 83% 83% 40%

TABLE 12 Example Downlink Budget Overview Value Units Link Budget InputParameters Information rate R_info 1.00E+07  bps 10 Mbps Code overheadeta_FEC 0.00% Codeless for now PPM order M 16 Number of slots per symbolLaser Transmitter Laser electrical input power PLD, elec 3.00 WElectrical input power Laser wavelength λpeak 1550 run Peak wave lengthElectro-optical efficiency η_TX 0.07 . Extinction ratio ER dB 27.04 dBUsed for “power robbing” correction. Half-power beam width θ 0.120 degPower is 0.5*peak full cone 2.09-1 mrad Transmit optical losses (dB) Lt, o pt, dB −−3.00 dB Real values from OCTL Channel Pathlength d_path1000 km LEO at 400 km −+ 20 deg above horizon Atmospheric loss Latm, dB−−1.00 dB Pointing loss Latm, dB −−3.00 dB Receive Telescope & OpticsFocal length FL 3 M Aperture diameter d 30 cm Receive optics lossesL_RX, optics −3.00 dB Background Noise (Sky Rad.) Field of view (fullcone angle) 6.67E−−05 rad PPM: C30662EH has 0.2 mm diameter Sky SpectralRadiance L_sky 6.00E−−04 W (cm{circumflex over ( )}2 + S Daytime • 2 kmabove sea level at 975 nm [Hemmati FIG. 8.16] Optical filter band″idth Bopt 1 nm Receiver Electronics Module Responsivity (w/gain) R_V_per_W340,000 V/W Transimpedance R fb 68,000 ohm Approximated fromresponsivity curve Excess noise factor F 5.5 Noise equivalent power NEP6.50E−−14 W/sqrt(Hz) W/sqrt(Hz) Electrical BW B 3.20E+07  Hz OOK: 0.8 *bit rate PPM: 0.8 * slot rate Liu Budget Summary Laser avg. opticalpower PLD, opt, avg, dB −−6.8 dBW Based on manuf specifications Transmitoptical losses (dB) L t, o pt, dB −−3.0 dB Placeholder Transmit antennagain(d•B) Gt, dB 65.6 dBi Uniform plane wave assumption. G = 16theta{circumflex over ( )}(Lambert eq3.78) Path loss (dB) Lpath, dB−−2581 dB Standard free-space path loss equation Atmospheric loss Latm,dB −−1.0 dB Placeholder Pointing loss Latm, dB −−3.0 dB PlaceholderReceive antenna gain (dB) Gr, dB 115.7 dBi Diffraction limited gainReceive optics losses L_RX, optics −−3.0 dB Placeholder Signal power atdetector P_sig, dB −−93.7 dBW Average received power Signal powerrequired P_req, dB −−97.1 dBW BER = le−−4 Margin for 1e−−4 BER 3.4 dB

Coarse Stage

The coarse stage of the system uses CubeSat body-type pointing (see,e.g., coarse pointing of FIG. 2 and corresponding description above). Insome embodiments, the coarse pointing sensors and actuators are notcontained within the FSO communications payload, but rather arecontained within or are part of the host CubeSat. CubeSats can use acombination of magnetorquers, reaction wheels, and/or thrusters forattitude control. Actuators that can slew include: thrusters,magnetorquers, reaction wheels, miniature control moment gyros, or anyother mechanism that can generate a torque. The FSO communicationspayload can be agnostic to the choice of sensors and actuators in thehost system. The host CubeSat has sufficient orbit and attitudeknowledge to initially point within 3° (i.e., initial pointing accuracyof 3°) of the ground station (e.g., based on the field-of-view of thebeacon receiver). The CubeSat can be configured to autonomously slewwith respect to inertial space to achieve initial alignment of thebeacon receiver with the optical beacon (e.g., in advance of the CubeSafs pass of a ground terminal, such that the lasercom beacon camera isstaring at the point on the horizon where the terrestrial station willappear).

Once an initial alignment is achieved and/or the beacon has beenlocated/acquired, the CubeSat can slew with respect to theground/terrestrial terminal at a slew rate of 1° per second, or up to1.1° per second (e.g., orbit dependent, to slew to track the terrestrialterminal through the pass). The beacon can provide very fine attitudeknowledge, approximately 30 μrad, and the CubeSat undergoes a transitionto become actuation-limited (during tracking). At this point, the hostCubeSat points to within 1° of accuracy to overlap with the pointingrange of the fine stage. The FSO communications payload, which containsa FSM for fine steering, can then “dial in” the transmitter to thedesired accuracy for downlink. A distinction between attitudedetermination and orbit (position) determination can be made, in thatposition/orbit determination is relevant to both ends of the link. Forexample, position/orbit determination impacts how the ground stationpoints the uplink beacon laser. For the satellite, orbit determinationalong with attitude determination impact satellite pointing.

Fine Stage

With regard to the fine steering mechanism (see, e.g., finepointing/steering of FIGS. 2-3 and corresponding description above),component selection criteria can include (but are not limited to): fieldof regard, accuracy, bandwidth, size, weight and power (SWaP)considerations (e.g., as affected by a mirror and its driver). In someembodiments, a MEMS fine-steering mirror (“FSM”) is used (e.g., having amechanical resonance of 430 Hz), and a Bessel filter can be employed toprotect the FSM. The MEMS fine-steering mirror can comprise a 2-axisMEMS tip/tilt mirror (e.g., a Physik Instrumente S-334 piezoelectricallyactuated tip/tilt mirror or a Mirrocle Technologies, Inc. S1630DBgimbal-less two-axis scanning MEMS micromirror) with a steering range(e.g., of about +/−1.25° or about +/−2.86° or about +/−5.73°), a size(e.g., of about 1.25 mm, or of about 4.2 mm), and a bandwidth (e.g., ofabout 300 Hz, or of up to 200 Hz, or of up to 1kHz). The MEMSfine-steering mirror can be disposed within a small, chip-scale packagehaving no integrated feedback sensors. Qualification parameters for afine steering mechanism can include positioning repeatability, thermalstability, and/or the ability of a mirror to be driven open-loop.

The fine-pointing module can be configured to point the opticaltransmitter toward a remote terminal with an accuracy range thatoverlaps with an accuracy range of the coarse-pointing module of theCubeSat. In some embodiments, overlap is desired because, for example,if the CubeSat can only get to within 3° accuracy for example, and theFSM can only reach 1° at the edge of its motion, it would be difficultor infeasible to apply an error correction. As such, in someembodiments, the fine stage range is driven by the actuation-limitedCubeSat pointing capability. In some embodiments, the cubesat payloaddoes not include an electromechanical gimbal. The gimbal is replaced bythe fine steering mirror combined with the fact that it is generallyacceptable to body slew a CubeSat.

Specifications for the accuracy of the fine stage can be based on adetailed link budget analysis to size the beamwidth of the CubeSatpayload. With a beamwidth of 2.1 mrad, the 3-pointing accuracy is set asa quarter of the beamwidth, e.g., 525 μrad(0.03°). The pointing loss isthus limited to 3 dB in the worst case. Therefore, the fine stageprovides a range of 1° to overlap with the CubeSat body pointing and afinal accuracy of 525 μrad. The combination of the coarse and fine stagecontrol can achieve a pointing accuracy of ±90 μrad, excludingconsideration of pointing bias. This gives approximately 7dB of marginover ±525 μrad. In the worst case scenario (i.e., the worst possiblepointing that still meets the requirements described herein), thepointing loss is maintained within 3dB.

To characterize the fine pointing stage, a 650 nm red laser was directedthrough a focusing lens, and steered into a Si camera by a FSM. Theangle of the FSM was determined based on the geometry of the setup.Since there was no feedback available on the device's position, it wasnecessary to characterize repeatability of the device to ensure that itcould meet performance requirements. To test repeatability, the mirrorwas commanded to visit each of the points in a 5-sided die patterncovering its entire range. For each iteration, points were visited in arandom order. Statistics on the position repeatability for a significantnumber of trials (N=500) show that the RMS error of the device is 12μrad, well within the desired performance.

Beacon Design

With regard to the beacon (see FIG. 1 and corresponding descriptionabove), design drivers can include (but are not limited to): satelliteposition uncertainty, eye safety (e.g., ANSI Z136.6, “Safe Use of LasersOutdoors,” NASA Use Policy for Outdoor Lasers, FAA Regulation: Order JO7400.2), and/or detector technology. In some embodiments of thedisclosure, a broad, near infrared uplink beacon is used, and isintercepted by a focal plane array on the satellite (i.e., the CubeSat).Beacon system analysis can include atmospheric fading, detector noisemodeling and centroiding algorithm performance. As described herein, thebeacon system can provide an average attitude knowledge accuracy of 30μrad with 2.3% fading probability in each frame read-out.

CubeSat FSO Communications System—Operation

FIGS. 4A-4C illustrate a sequence of a pointing, acquisition, andtracking (“PAT”) process executed by a CubeSat with fine a coarsepointing stages. During operation, a host CubeSat 400 autonomously slewsfrom a mission-defined attitude. In FIG. 4A, an acquisition sensor(e.g., having a FOV 401 of 6.6°×8.7° (full-angle)) of an FSOcommunications module of the host CubeSat 400 stares for a beacon signalB with a coarse ADCS accuracy of ˜2° (e.g., based on attitude knowledgeand/or position knowledge). The beacon can originate from a remoteterminal 404 or from its general vicinity. In systems where uplink anddownlink wavelengths are similar, it can be advantageous to provide somespacing between the beacon transmitter and the downlink receiver inorder to reduce noise caused by scattered light from the uplink.

In FIG. 4B, a centroid algorithm, running on the FSO communicationsmodule, estimates a boresight offset, and the ADCS subsystem of the hostCubeSat closes control loops using a beacon offset. The beacon beamdivergence is ˜5 mrad full width at half maximum (“FWHM”), whichaccommodates a tracking error of less than +/−1 km. The point-aheadangle in LEO is orders of magnitude smaller than the specifiedbeamwidth. As such, the point-ahead angle is ignored due to beamwidthand orbit geometry (10 arcsec (51 μrad)). In FIG. 4C, an integratedfine-steering mechanism rejects residual error, and coarse correctionsare fed to the host ADCS. Although some degree of range, resolution,and/or bandwidth limitation is inherent to all actuators and sensors,multi-stage solutions (i.e., the staged control approach describedherein) can alleviate such limitations.

Tables 13 and 14 (below) provide exemplary overviews of the coarse andfine stages of a staged control approach to PAT, according to someembodiments.

TABLE 13 Overview of Coarse Stage (Host CubeSat) Requirements ParameterRequirement Initial pointing accuracy ±3° Actuation-limited pointingaccuracy ±1° Max. slew rate up to 1.1°/s (orbit dependent)

TABLE 14 Overview of Fine Stage Requirements Parameter Requirement Range±10 Pointing accuracy 525 μrad (0.03°)

FIG. 5 is a rendering of an FSO satellite communication system accordingto some embodiments. An FSO module on board the satellite 500 has a datarate of about 10 Mbps to about 50 Mbps, with comparable powerconsumption as compared with existing RF solutions. While the high-ratedownlink and beacon uplink are both optical, the system also uses alow-rate RF uplink and downlink 501 (via RF station 502) for high-levelcommand and control and limited data downlink, for example duringperiods of optical link unavailability, should they occur. The RF link501 can be supported with minimal resources in terms of licensing,ground systems, and power.

As shown in FIG. 5, an uplink optical beacon B (e.g., originating fromoptical station 504), having a wavelength of about 850 nm, is used foracquisition and tracking, and an optical downlink beam 503 at about 1550nm is used for high-rate data transmission. The transmitter opticalpower budget can be a function of the downlink data rate and/or thesize, cost and/or the portability of the ground terminals. A link budgetanalysis can be performed to determine a target transmit beamwidth(e.g., 2.1 mrad) as well as other PAT considerations for minimizingpointing losses to acceptable levels. In some embodiments, thetransmitter is configured to output (e.g., via an EDFA) high-fidelitywaveforms with an extinction ratio (“ER”) of >about 33 dB, at anelectrical power margin of about 18% and a modulation bandwidthof >about 600 MHz (e.g., FPGA-limited).

FIGS. 6, 7 and 8 are renderings of first, second, and third steps(respectively) in a PAT process implemented using the system of FIG. 5.During the first step (FIG. 6), the CubeSat 600 slews toward a groundstation 604 or other remote terminal (i.e., seeking to acquire an uplinkbeacon emanating therefrom) using one or more coarse sensors and one ormore reaction wheels as actuators, with a pointing accuracy of +/− about3°. Coarse pointing can be based on two-line element (“TLE”)information. Two line element sets are published by the Joint SpaceOperations Center for satellite operators to utilize. Based on thesatellite's orbital position from the TLE and the known location of theground station, the satellite can point towards the ground station.

During the second step (FIG. 7), the CubeSat 600 closes its control looparound a beacon offset using a beacon camera and one or more reactionwheels as actuators, with an improved pointing accuracy of +/− about1.25°. The beacon camera determines (“sees”) the location of the beaconand, correspondingly, where it should point to achieve a high degree ofaccuracy (the accuracy being limited, in some embodiments, by theactuators' capabilities). A beacon camera can comprise a CMOS focalplane array (e.g., 5 megapixels) such as an Aptina MT9P031 (e.g., havingan optical format of 1/2.5″, a resolution of 2,592H×1,944V, a pixelpitch of ˜2.2 μm, and a quantum efficiency (“QE”) at 850 nm of ˜15%), alens systems (e.g., 1″, f=35 mm), one or more bandpass filters to rejectbackground light, and/or one or more UV/VIS-cut filters to reduce systemheating.

During the third step (FIG. 8), the fine steering mechanism of theCubeSat 600 is activated. This step continues to use the beacon camerafor sensing, but transitions to using a fast-steering mirror as itsactuator, with a pointing accuracy of +/− about 0.03°. The CubeSat canthen steer its downlink based on a beacon boresight offset. Exampleparameters for the transmitter shown in Table 15 below. Exampleparameters for the beacon camera optics are shown in Tables 16 and 17below.

TABLE 15 Transmitter Design Parameters Parameter ValueJustification/Driver Optical >200 mW avg Link budget, PPM-16 outputpower assumed Modulation PPM, M = [8-64] ER implications typeModulation >1 GHz desired To support future pointing BW improvementsWavelength ±1 nm Ground receiver filter stability Operating 0° C. to 40°c. Typical CubeSat values temp. range (inside chassis) Input power <8 WTransmitter portion of terminal Size goal <10 cm × 10 cm × 3 cmTransmitter portion of terminal Mass goal <300 g Transmitter portion ofterminal

TABLE 16 Beacon Camera Parameters Parameter Value Detector resolution2592 H × 1944 V Pixel's pitch 2.2 μm Focal length 35 mm Field of view 7°850 nm band-pass filter bandwidth 10 nm Long-pass filter cut-offfrequency 700 nm

TABLE 17 Beacon Camera Parameters Lens + filters Focal length 35 mmAperture 1″ Band-pass filter (850 +/− 5) nm Long-pass filter >700 nm

In some embodiments, a field-programmable gate array (“FPGA”) is usedfor transmitter modulation.

In some embodiments, the beacon receiver camera comprises a CMOS focalplane array, a 1″ aperture lens system, and two optical filters. Thedetector is configured to have a high NIR sensitivity, resolution andlow dark current and read noise properties. The lens system isconfigured to provide a wide effective field-of-view (7°) that cansufficiently compensate for the satellites pointing capability with onlycoarse sensors. Two optical filters are used: a bandpass filter at 850nm and a UV/VIS-cut filter to reduce heating caused by Sun radiation.The beacon camera system size is approximately 4 cm×4 cm×6 cm with atotal weight of 160 g, and can include a UV/VIS-cut filter to reducesystem heating.

FIG. 9A is a rendering of a closed-loop tracking configuration for anFSO communications module, using a quadcell tracking detector 923 a,according to some embodiments. A single aperture 916 is used fortransmitted light (passing from the transmit laser 915 a through beamsplitter 912, then reflecting off of the FSM 917 a and directed throughthe aperture 916) and received light (reflecting off of the FSM 917 aand directed to the beam splitter 912, which diverts the incoming lightonto the quadcell tracking detector 923 a). Quadcell tracking detector923 a signal data is processed by PAT circuitry 925 a. Transmit laser1015 a is driven by high speed electronics 907 a, and the FSM 917 a isdriven by high-voltage drive electronics 927 a. As compared with thefocal plane detector configuration discussed below with reference toFIG. 9B, the quadcell configuration has a narrower field of view (“FOV”)and more complex optics, but has a higher sensitivity.

FIG. 9B is a rendering of a closed-loop tracking configuration using afocal plane tracking detector, according to some embodiments. Separateapertures (941 and 943) are used for transmitted light (passing from thetransmit laser 915 b to the FSM 917 b where it is reflected and directedthrough aperture 941) and received light (passing through uplinkaperture 943 and incident directly onto the focal plane trackingdetector 923 b), respectively. Focal plane tracking detector 923 bsignal data is processed by PAT circuitry 925 b. Transmit laser 915 b isdriven by high speed electronics 907 b, and the FSM 917 b is driven byhigh-voltage drive electronics 927 b. As compared with the quadcelldetector configuration discussed above with reference to FIG. 9A, thefocal plane configuration has a wider FOV and simpler optics, but isless sensitive.

Ground Segment

FIG. 10 is a system block diagram showing components of a ground segmentaccording to some embodiments. Control electronics 1045, configured toreceive ephemeris data, are electrically coupled to a beacon laser andamplifier 1047 whose collective output is optically transmitted througha beacon aperture 1049 (e.g., about 10 cm in diameter). The controlelectronics 1045 are also electrically coupled to an azimuth-elevation(“Az/El”) mount 1046 that is mechanically coupled to the beacon aperture1049 as well as to a receive aperture 1051 (e.g., about 30 cm indiameter). The receive aperture 1051 is configured to pass an incomingoptical transmission to an optical detector 1053, whose output iselectrically fed to a high-speed electronics module 1067. The high-speedelectronics module 1067 contains a sequence of functional blocks: ademodulator 1055, a framer 1057, an interleaver 1059, and an encoder1061. The output of the encoder is checked using a pseudorandom binarysequence (PRBS) checker 1063, and is optionally (e.g., if no or lowerror is found) stored to a mass storage unit 1065. Data in the massstorage unit 1065 (e.g., a downlink request) can then be retrieved usingone or more transport protocols 1068 (e.g., automatic repeat request(“ARQ”)) via the low-rate RF TT&C link 1009.

Transmitter Selection

A radiometric link budget analysis was performed to estimate the opticaltransmit power to close a 10 Mbps link for a CubeSat implementation.This analysis was constrained by the expected pointing capability of thespace segment/terminal (e.g., which sets the downlink beam divergence to2.1 mrad FWHM), the link range (e.g., <1000 km) as well as thesensitivity of the ground receiver (e.g., 1000 photons per bit, allowingfor the use of COTS detectors), and indicated that approximately 1W ofoptical power would close the link at a 10 Mbps user data rate in areceiver thermal-noise-limited system. At 1 W power levels, two opticalsources were identified as candidates for the system: a high power laserdiode (HPLD), such as a “pump” laser at 980 nm, and a master-oscillatorpower amplifier (MOPA) design incorporating a fiber amplifier at either1μm or 1.55 μm. The effectiveness of each of these configurations, basedon end-to-end link performance, is discussed below. This performanceanalysis incorporated realistic transmitter assumptions (e.g. modulationtype) and receiver parameters (e.g. suitable detector technology fortransmitter wavelength). System parameters were matched where possible,and notable differences are enumerated in Table 18.

TABLE 18 Differences in HPLD and MOPA system parameters Option A OptionB HPLD MOPA Wavelength 980 nm 1550 nm TX power (avg) 500 mW 200 mWModulation OOK PPM-16 Receiver BW Per modulation specifications DetectorSi InGaAs APO/TIA APO/TIA Performance Modulation Wall-plug Limiterbandwidth power

FIG. 11 is a block diagram showing components of a HPLD configuration,suitable for use in the optical transmitters of FIGS. 2-4. The HPLDconfiguration consists of a directly modulated high-power laser diode,such as a 980 nm “pump” laser diode 1122. These devices can be obtainedin convenient single mode fiber-coupled butterfly packages. Theelectrical-to-optical (EO) conversion efficiency of HPLD lasers istypically greater than 30%. Operation at 980 nm is also advantageousfrom a receiver perspective, as silicon detectors are near their peakresponsivity at this wavelength. Conditioned power (e.g., DC/DCconversion at 1118) is supplied to a high-current driver circuit 1120that drives the pump laser 1122, and the laser output is fiber-opticallyrouted to collimation optics. A disadvantage of the HPLD approach stemsfrom the associated driver circuitry, which switches large amounts ofcurrent at the modulation bandwidth. Assuming on-off-keying (OOK), whichminimizes modulation bandwidth relative to data rate, and a typical pumpdiode efficiency (η=0.6 W A⁻¹), the driver circuit would switch over˜1.5-2 A at 10 MHz rates. This approach is feasible, and is used in somesystems (e.g., laser video projection systems), but may be fundamentallylimited by the packaging of the laser. Nevertheless, this configurationis well within power budget: 3.3W estimated of 8W budget and anestimated wall-plug efficiency of 15%.

FIG. 12 is a block diagram showing components of a MOPA configuration,according to some embodiments, suitable for use in the opticaltransmitters of FIGS. 2-3. Drive circuit 1269 provides FPGA directmodulation, bias, and TEC control, and is electrically coupled to seedlaser 1271. Alternatively or in addition, the drive circuit 1269includes one or more laser drivers and/or one or more thermoelectriccooler (“TEC”) drivers. Seed laser 1271 comprises a transmitter opticalsubassembly (“TOSA”) module including a distributed feedback (“DFB”)laser diode and a TEC (which may be part of the TOSA module). Outputfrom the seed laser 1271 is optically coupled to an extinction filter1273 (e.g., an athermal FBG filter, circulator, bandpass spectralfilter, etc.), for example to improve the extinction ratio (“ER”) of theseed laser signal through FM-to-AM conversion. Precise alignment betweenthe seed laser wavelength and the filter passband can simultaneouslyachieve high ER and low insertion loss. In some embodiments, athermalfiber Bragg grating (“FBG”) filters can provide both steep transitionregions (>>1 dB/GHz) and high stopband attenuation (>30 dB) along with athermally stable center wavelength (≈100 MHz/° C.). A temperature sensorcan be mounted to a FBG filter and used to compensate for the slightthermal dependency of the FBG filter. The overall wavelength shift ofthe transmitter during such compensation can be acceptable, for examplewhen the ground station receive filter bandwidth is 250 GHz (2 nm).

The filtered signal (i.e., the output of extinction filter 1273) is thenfed to an amplifier. The MOPA configuration uses anaverage-power-limited fiber amplifier such as an Erbium-doped fiberamplifier (“EDFA,” e.g., 1.55 μm) 1275 a or an Yttrium-doped fiberamplifier (“YDFA,” e.g., 1 μm), and is amplified by EDFA 1275 (e.g.,with a gain of ˜40dB). Average-power-limited amplification allows thesystem to take advantage of low duty-cycle waveforms with highpeak-to-average ratios such as pulse position modulation (“PPM”).Although YDFAs can provide roughly twice the wall-plug efficiency ofEDFAs, these efficiency levels can be difficult to realize at lower(<1W) power levels, and may be less commercially available. For purposesof analysis herein, a 200 mW “MSA” form-factor EDFA that is compatiblewith the volume constraints of CubeSats was used as a baseline. Thelower output power of the MOPA (relative to the HPLD) is roughlybalanced by the link margin gains afforded by moving to PPM from OOKmodulation. The EDFA 1275 amplifier output is fiber-optically routed tocollimation optics.

The EDFA 1275 amplifier output is then fiber-optically routed tocollimation optics. In some embodiments, a MOPA transmitter produceshigh fidelity PPM waveforms at 1550 nm at 200 mW average output powerwhile consuming 6.5 W of electrical power.

Aside from the amplifier, the modulator (e.g., see 215 in FIG. 2 andcorresponding description above) is typically a large power consumer inMOPA designs. For PPM waveforms, the modulator can provide highextinction ratio (“ER”) in order to avoid “power robbing” losses in thefiber amplifier (e.g. for PPM-16, ER>27dB). To avoid the power penaltyassociated with an external modulator, a direct modulation (“directlymodulated laser,” “DML”) approach can be used. The transmitter digitalelectronics (e.g. an FPGA) can directly modulate the seed laser with thecommunication waveform. Direct modulation alone can provide 10 dB of ERif the laser is kept above a threshold (ith). To further improve ER, thelaser's adiabatic frequency chirp can be used in conjunction with anarrow bandpass filter to produce FM-to-AM conversion. The MOPA designis estimated to consume about 6.5W, yielding a wall-plug efficiency ofapproximately 3%. This is lower than the HPLD design, however the MOPAis capable of producing higher fidelity waveforms at much fastermodulation rates. MOPA designs can have high modulation bandwidths, highpeak-to-average power ratios, good component availability and a cleanspectral output.

Seed-to-EDFA power budget data for MOPA architectures according to someembodiments is provided in Table 19 below.

TABLE 19 Seed-to-EDFA Power Budget (e.g., EDFA driven into saturation)Parameter Value Notes/Justification Seed laser power output −4 dBmAverage power (+12 dB for peak at M = 16) Circulator Loss −1.2 dB FBGFilter Loss −1.3 dB Connector/coupler losses −0.5 dB Conservative budgetfor flight design EDFA Gain 40 dB EDFA avg output power +23 dBm Margin10 dB EDFA driven 10 dB beyond saturation

Comparisons of the HPLD and MOPA configurations are provided in Tables20-21 below. Both HPLD and MOPA configurations gave a >3dB link marginfor a 10 Mbps data rate at a specified bit error rate (“BER”) of 1×10⁻⁴(uncoded).

TABLE 20 Comparison of HPLD and MOPA Architectures Parameter HPLD MOPAWavelength 980 nm 1550 nm Approx. Size 5 cm × 5 cm × 1 cm 10 cm × 10 cm× 3 cm Approx. Mass 100 g 250 g Approx. Power 3.3 W 6.5 W ModulationBandwidth <50 MHz (package >1 GHz parasitics) Peak-to-average Limit Low(typ. <10) High (>16) Spectral Quality Poor (>1 nm) Excellent NotableRisks Driver circuit design Wall-plug power Spectral quality Achievinghigh ER

TABLE 21 Comparison of HPLD and MOPA Architectures Config Config A B(Direct) (MOPA) Units Notes Laser avg. optical −3.0 −6.8 dBW Manuf.power specifications Transmit optical losses −3.0 −3.0 dB TBR Transmitantenna gain 65.6 65.6 dBi 2.0 mrad divergence Path loss −262.2 −258.2dB Free-space path loss Atmpheric loss −1.0 −1.0 dB TBR Pointing loss−3.0 −3.0 dB TBR Receive antenna gain 119.7 115.7 dBi Diffractionlimited gain, 30 cm Receive optics losses −3.0 −3.0 dB TBR Signal powerat −89.9 −93.7 dBW detector Signal power required −92.7 −97.1 dBW For 10Mbps at BER = 1e−4 Margin for le−4 BER 2.8 3.4 dB

PAT Analysis

Analysis of a PAT attitude control system according to some embodimentswas performed using a single-axis tracking simulation. FIG. 13 is adiagram showing the linear single-axis model used, and Table 22 belowshows the simulation parameters used. A spacecraft 1300 was modeled toinclude a reaction wheel 1324, gyroscope 1326, FSM 1317, and FPAdetector 1323, and to have a disturbance torque applied thereto.Simulated beacon position information was routed from detector 1323 to acentroiding/attitude determination module 1328, which determined anattitude error and passed it to a low pass filter 1332, an attitudecontrol module 1334, and the Kalman filter 1330. Attitude correctiondata was routed from the low-pass filter 1332 to the fast-steeringmirror 1317. The attitude control module 1334 received angular rate datafrom the Kalman filter 1330, as well as reference attitude data (e.g.,from memory) to calculate a command torque which was then passed toreaction wheel 1324 as well as back to the Kalman filter 1330 input.Angular rate information was routed from gyroscope 1326 to a Kalmanfilter 1330. Attitude dynamics were expressed as follows:

${\begin{matrix}{x = \begin{bmatrix}\theta \\\theta\end{bmatrix}} & {\overset{.}{x}\begin{bmatrix}0 & 1 \\0 & 0\end{bmatrix}}\end{matrix}x} + {\begin{bmatrix}0 \\\frac{1}{J}\end{bmatrix}\tau_{cmd}} + {\begin{bmatrix}0 \\\frac{1}{J}\end{bmatrix}\tau_{dist}}$

The feedback controller was a PID controller run at 4 Hz (based on theMAI-400 reaction wheel), and gains were selected for a damping ratio of0.7 and crossover frequency of 0.04 Hz:

$\begin{matrix}\begin{matrix}{K_{P} = {J\; \omega_{n}^{2}}} & {K_{D} = {2J\; {\eta\omega}_{n}}}\end{matrix} & {K_{I} = \frac{K_{P}}{10}}\end{matrix}$

where K_(P) is proportional gain, K_(D) is derivative gain and K_(I) isintegral gain.

TABLE 22 Simulation Parameters Value Optics Focal Length 35 mm CentroidError 30 μrad RMS (0.5 pixel) Reaction Wheel (MAI-400) Max. Torque 635mNm Resolution 0.005 mNm Gyroscope (ADIS-16334) Angular Random Walk 2deg/√hr Output Noise 0.75 deg/s RMS Resolution 0.0125 deg/s SpacecraftTrue Moment of Inertia 0.05 kgm² Est. Moment of Inertia 0.0475 kgm²

During the PAT analysis, the performance of a system using only a coarsestage was compared with the system using both coarse and fine stages. A400km altitude, an acquired beacon, and environmental disturbances suchas solar radiation, magnetic interference, a gravity gradient, andaerodynamic drag were simulated. The combined “coarse and fine stage”pointing significantly outperformed the exclusively coarse stagepointing (i.e., the attitude error was considerably lower for thecombined “coarse and fine stage” as compared with the exclusively coarsestage pointing), and fell well within the limits of an exemplarydownlink error range requirement. The estimated coarse pointing accuracywas +/−1.6 mrad (3-σ), while the estimated fine pointing accuracy was+/−80 μrad (3-σ).

ADCS Parameters

FIG. 14 is a flow-down diagram of parameters of attitude determinationand control systems. Uplink parameters include uplink beacon size/power1446 (e.g., interdependent upon COTS coarse pointing component 1440selection) and detector selection 1448 (e.g., based upon COTS coarsepointing component 1440 selection, ephemeris knowledge 1450 and/orground station tracking error 1452). Downlink parameters include finestage FSM selection 1442 (e.g., interdependent upon COTS coarse pointingcomponent 1440 selection) and acceptable pointing losses 1444 (e.g.,interdependent upon the fine stage FSM selection). COTS coarse pointingcomponent 1440 selection can depend upon maximum slew rate 1436 (e.g.,˜1°/s) and/or pointing error 1438 (e.g., <5°).

FIG. 15 is a flow-down diagram of constraints of attitude determinationand control systems. 3U CubeSat SWaP constraints 1554 (an externalconstraint) are considered when identifying self-imposed FSO payloadSWaP limits 1560. The FSO payload SWaP limits correspond to an availablepower that affects the FSO beam width 1566 (i.e., a derived constraint).The RF communications infrastructure 1556 used (e.g., an externalconstraint) is a factor in determining a target FSO link acquisitiontime 1562 and FSO link rate 1564. The FSO link rate also affects the FSObeam width 1566 (a derived constraint). The existing CubeSat ADCStechnology 1558 used, and its associated coarse pointing capability (anexternal constraint), together with the FSO beam width 1566, impact thefine pointing capability 1568 that can be achieved.

FIG. 16 is a transmitter test configuration according to someembodiments. Support equipment includes a laser diode controller 1670(e.g., ILX Lightwave LDC-3724C), a modulator 1672 (e.g., FPGA evaluationboard), an optical power meter 1692, a high bandwidth optical detector1694, and an oscilloscope 1696. Seed laser 1610 includes TEC 1674,thermistor 1676, laser diode 1686 and photodiode 1688. The laser diodecontroller 1670 provides bias current to the laser diode 1686, and isalso electrically coupled to TEC 1674 and thermistor 1676. Modulator1672 applies a PPM-16 modulation to the laser diode 1686, whose outputis fed through a coupler to both “test port 1” and to a 3-portcirculator 1684 in optical communication with an FBG filter 1690. Outputfrom the 3-port circulator 1684 is fed through a coupler to “test port2” as well as to EDFA 1682 (powered by 5V DC power supply 1678 andcontrolled by control PC 1680 via an RS232 connection). The EDFA 1682output is optically fed to “test port 3.” The test configuration of FIG.16 was used to produce the results in FIG. 17 (laser current andtemperature wavelength tuning), FIG. 18 (thermal stabilization powerconsumption), FIG. 19 (filter characterization), FIG. 20 (extinctionratio validation) and FIGS. 21A-F (EDFA validation), discussed ingreater detail below.

Seed Laser Selection and Characterization Example

In some embodiments, selection criteria for a seed laser (e.g., for theMOPA configurations discussed above with reference to FIG. 12) includepower consumption of the integrated thermoelectric cooler (TEC), sizeand/or mechanical layout. Accurate temperature control is necessary tostabilize the laser wavelength, and on many lasers the TEC requires asignificant amount of power (>1 W). TOSAs can include low power TECs(<0.4 W), and are available in very compact, fiber-coupled packages(e.g., 20 mm×8 mm×5 mm). In some embodiments, a TOSA has 6 dBmcontinuous wave (“CW”) output power in the 1550 nm C-band.

An automated testbed comprising a laser diode controller, a wavemeterand an optical spectrum analyzer was used to characterize tuningparameters of a seed laser according to some embodiments (see FIG. 17,showing a plot of wavelength versus DC laser bias, with each curvecorresponding to a different seed laser temperature). The approximatetuning characteristics for a representative device were Δλ/ΔT=−11 GHz/°C., Δλ/Δi_(DC)=−0.45 GHz/mA, and Δλ/Δi_(AC)=0.20 GHz/mA (measured withPPM at f slot=200 MHz). The TOSA's TEC power consumption was validatedby measuring both VTEC and iTEC while the setpoint (Tset) was adjustedrelative to ambient. Details relating to the transmitter power budgetare provided in Table 33 below.

TABLE 23 Transmitter Power Budget Value Notes EDFA 5.7 W Manuf. worstcase, (we measured: 4.1 W) Seed laser TEC 0.4 W (TBRR) Peak power, overtemp Seed laser DC bias 0.2 W Worst case Seed laser AC drive 0.01 W 50mA, 1/16 duty FPGA logic 0.2 W Only TXer related portion of FPGA Total:6.51 W Margin: 1.49 W 8 W budgeted

FIG. 18 shows a plot of the measured seed laser TEC power consumptionversus temperature for an exemplary transmitter, as well as a quadraticmodel for both heating mode (right half of curve) and cooling mode (lefthalf of curve). The TEC power consumption was within budget (0.4W), with18% margin, across the expected operational range for the device (0 ° C.to 40° C).

FIG. 19 is a plot of insertion loss versus frequency, comparing theinsertion loss of two Gaussian passband athermal fiber Bragg grating(“FBG”) filters (having bandwidths of 5 GHz and 10 GHz, >40 dB stopbandand |Δλ/° C.|<˜125 MHz/° C.), along with the expected wavelength “chirp”(Δλ/Δi_(AC)) of the seed laser, according to some embodiments. The 10GHz chirp shown in FIG. 23 was achieved with 50 mA of direct drivecurrent from a Xilinx Spartan 6 FPGA LVCMOS I/O pin. Only the narrower 5GHz FBG filter was able to produce sufficient seed suppression whilemaintaining low insertion loss. In some embodiments, temperature/biaswavelength tuning aligns the seed laser with the filter, and signalmodulation ER is enhanced through FM-to-AM conversion.

A swept duty-cycle ER measurement (with peak power variationcompensation applied) revealed that the combined seed laser, incombination with an extinction filter, was achieving an ER of >33 dB atslot frequency f_(slot)=200 MHz (see FIG. 20, a plot showing extinctionratio measurements for a seed laser with an extinction filter, accordingto some embodiments). Slot frequency is the rate of PPM slots. In otherwords, 1/(slot frequency) is the duration (or “pulse length”) of thetransmitter pulses. The aforementioned ER is sufficient for lowduty-cycle waveforms, such as 64-ary PPM. The same measurement wascompleted at f_(slot)=40 MHz, and showed a slightly degraded ER (≈28dB). Without wishing to be bound by theory, this is believed to be dueto fact that the modulation frequency is closer to the transition pointwhere thermal effects begin to dominate charge carrier density effects.The higher modulation rate may have had better extinction due to ahigher value of |Δλ/Δiac|. In some embodiments, the transmitter isoperated at f_(slot)≧200 MHz, and the modulation order (M) is varied toachieve a variety of link rates.

FIGS. 21A-21F show an FPGA, seed laser +ER filter, and EDFA sequence,with related electrical and optical input/output time-domain waveformplots, according to some embodiments. The time-domain measurements showthat the design produces high-fidelity optical waveforms, as shown inFIGS. 21B, 21D and 21F. Furthermore, the >33 dB ER of the combined seedlaser and filter assembly allows operation at high order PPM (64-ary)without sacrificing the peak power gains offered by theaverage-power-limited EDFA. Since the design supports modulationbandwidths in excess of 600 MHz, it will be suitable for use in futuresystems with enhanced pointing capabilities. The power consumption ofthe transmitter is within budget with nearly 20% margin (see Table 24below).

TABLE 24 Transmitter Power Consumption Summary Transmitter powerconsumption summary Parameter Value Notes EDFA 5.7 W Manufacturer worstcase specification Seed laser TEC 0.4 W Peak power over 0° C. to 40° C.Seed laser DC bias 0.2 W Worst case Seed laser AC drive 0.01 W  50 mA,1/16 duty cycle FPGA logic 0.2 W Transmitter portion of FPGA (sharedwith other functions) Total 6.51 W  Margin 1.49 W  8 W budget fortransmitter

FIGS. 22A and 22B are Bode plots showing the frequency response of afine steering mirror, with and without a low-pass (e.g., Bessel) filter,respectively, according to some embodiments.

FIGS. 23A and 23B show power/performance plots for erbium-doped fiberamplifiers (“EDFAs”), according to some embodiments.

Simulation of Uplink Beacon Acquisition

Fine attitude sensing capabilities can depend upon the acquisition andtracking capabilities of the uplink beacon. Fading of the uplink beacondue to atmospheric turbulence can be of concern, especially with thehigh slew speed required to track the satellite in LEO (up to 1°/s). Assuch, a detailed analysis and simulation were performed to evaluate theperformance of a beacon system according to an embodiment.

Table 25 below presents a beacon uplink budget with a 10 W transmitter,5 mrad beamwidth when the satellite is at 20° and 90° elevation angle,with estimates of optical and atmospheric absorption and scatteringlosses (see Tables 25-27, also below). Noise components in thesimulation include shot noise from signal, background sky radiance, andthe noise sources of the beacon camera detector. In some embodiments,background shot noise is the dominant source. Various background lightconditions were analyzed including a worst case scenario consisting ofsunlit clouds. The estimated spectral radiance in this condition at 850nm is approximately 180 W/m²/sr/um. The estimated integration time isselected to maximize dynamic range given the well capacity of thedetector.

TABLE 25 Beacon Uplink Budget Beacon link budget Transmitter Transmitlaser power 10 W Uplink wavelength 850 nm Beam divergence 5 mrad Actualtransmit power 4 W 20° 90° elevation elevation Free-space/Atmosphericchannel Range 894 400 Km Atmospheric absorption and scattering −6 −5 dBReceiver Receiver bandwidth 10 10 Nm Average power at detector 0.0130.081 nW Interation time 1.6 0.5 Ms Total photons received 7.3E+041.8E+05 photons Noise level in ROI 7.0E+03 9.9E+03 photons Optical S/N10.1 12.5 dB

TABLE 26 Link Analysis Transmit power 10 W Wavelength 850 nm Beamwidth 5mrad Range (20° elevation) 984 km Atmospheric −6 dbabsorption/scattering Sky radiance⁵ 180 W/m³/sr/um Receiver bandwidth 10nm Optics loss (Tx + Rx) −8 db Received power 0.013 nW Margin 10 db

TABLE 27 Scintillation Statistics$C\mspace{14mu} \frac{2}{n}\mspace{14mu} {profile}$ Huffnagel-Valieymodel³ 1°/s slew speed Scintillation index Strong-turbulence model³Spatial diversity (4 beams) Distribution Log-normal

The atmospheric refractive index structure parameter (C² _(n)) profilefor a mission can be estimated using the Hufnagel-Valley model. Sinceuplink beam will be slewing up to 1°/s to track the satellite in LEO,the slew rate becomes the dominant “wind-speed” parameter. This slewrate was incorporated in the Hufnagel-Valley model as additional windspeed through the Bufton wind model, leading to a more turbulent C² _(n)profile than the standard HV5/7 profile, as seen in FIG. 24 (a plotshowing atmospheric refractive index structure parameter profiles for astationary beam and for a beam with 1°/s slew speed). The scintillationindex can be estimated using the strong-turbulence model. A highscintillation index, e.g. caused by a fast slew rate, can be reduced bythe use of multiple independent transmitters for spatial diversity. Fora satellite at 400 km altitude, the scintillation index of the uplinkchannel with 4 independent transmitters is approximately 0.3. Signalpower fluctuations about the mean value from the above link budget canbe estimated using a log-normal distribution with variance equal to thescintillation index. Additional information on the models referencedherein (e.g., Hufnagel-Valley model and Bufton wind model) can be foundin “Laser Beam Propagation through Random Media,” second edition, byLarry Andrews and Ronald Phillips (2005).

The fade probability and centroiding accuracy were found by running thesimulation with scintillation statistics with a time series of expectedbeam motion at 20° elevation from a 400 km altitude orbit. A fadeinstance is defined as the case where the brightest pixel does notbelong to the beacon image on the detector array. In this simulation,the scintillation time scale is assumed to be comparable to thedetectors integration time. The fade probability indicates theprobability that the beacon is not found within the time it takes toread out a frame, which is approximately 0.15 s for a 5 megapixelcamera. FIG. 26A shows the simulation results of fade probability atvarious transmitter power levels (5W, 7.5W, 10W). As shown in FIG. 26A,the fade probability can be reduced to about 2.3% given a 10 W transmitpower, sufficient for acquisition and tracking given proper real-timeestimation techniques.

The centroiding accuracy was found using center-of-mass centroiding ofthe beacon image on the detector (see FIGS. 25A and 25B). The accuracyresult when not fading is shown in FIG. 26B. The average centroidingaccuracy is approximately 0.5 pixel, corresponding to a mean attitudeaccuracy of 30 μrad. This accuracy result is less than 1/10 of thepointing requirement of the fine stage, leaving margin for actuationlimitations and errors.

Control System Simulation

A simulation of coarse and fine control stages was performed,incorporating the results of the uplink beacon simulation described inthe previous section. The accuracy of the detector was taken to be 30μrad on average. The rate at which the FSM can be driven is limited bythe beacon detector readout and processing. A readout rate of 10 Hz issufficient for an accuracy of 525 μrad, as shown in FIG. 31. Once thebeacon has been acquired, the CubeSat pointing performance can bedependent upon the control authority of its reaction wheels. The CubeSatmodeled in FIG. 27 is affected by torque quantization and is intended asa fairly low-performance actuator. Verifying that the fine stage canimprove performance to within required accuracy (with a margin of 7 dB)for this scenario indicates that it can meet the pointing requirementsfor a typical CubeSat.

Conclusion

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of the technology disclosed herein may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

The various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of” “only one of” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein, the terms “about” and “approximately” generally meanplus or minus 10% of the value stated.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

1. A Cubesat module for a CubeSat, the CubeSat module comprising: anoptical transmitter to transmit data to a remote terminal; a receiver toacquire an optical beacon from the remote terminal; and a fine-pointingmodule, operably coupled to a coarse-pointing module of the CubeSat, topoint the optical transmitter toward the remote terminal with anaccuracy range that overlaps with an accuracy range of thecoarse-pointing module of the CubeSat so as to establish acommunications link between the CubeSat and the remote terminal over alow-Earth-orbit (LEO) distance, wherein the coarse-pointing module isconfigured to point the CubeSat by slewing a body of the CubeSat.